1. Field of the Invention
The present invention relates to projection systems and projection screens and, more particularly, to a projection screen apparatus that provides improved image illumination uniformity.
2. Description of Related Art
Light projection is used to display images on large surfaces, such as large computer displays or television screens. In front projection systems, an image beam is projected from an image source onto the front side of a reflection-type, angle transforming screen, which then reflects the light toward a viewer positioned in front of the screen. In a rear projection system, the image beam is projected onto the rear side of a transmission-type, angle transforming screen and transmitted toward a viewer located in front of the screen.
Referring to FIG. 1, wide angle projection systems that include a screen apparatus 10 are known to optimally use a conventional Fresnel lens 11 in combination with some diffusing element, such as a substrate covered with glass beads (e.g., a type of diffuser or diffusive screen) 12. The combination forms an imaging screen that produces an image. The Fresnel lens 11 and the diffuser 12 are held in relatively rigid or semi-rigid spaced apart relation to assure proper operation of the combination. Such screens, known generally in the art as xe2x80x9cblack matrix beadxe2x80x9d or xe2x80x9cBMBxe2x80x9d screens, are commercially available from Minnesota Mining and Manufacturing Company and others. Fresnel lenses are sold by Fresnel Optics and are manufactured by Minnesota Mining and Manufacturing Company, for example, as used in devices such as overhead projectors. The Fresnel lens 11 element is constructed to provide the optical properties of a much thicker lens. however, with smaller size and weight. Concentric steps or discontinuities 11A allow these optical and physical properties to be realized. Each of the steps has a curved profile, in cross-section, that exhibits optical power to redirect incident light 13. The cut-out sections that define the steps reduce the overall size and weight.
In FIG. 1, the Fresnel lens 11 receives the incoming light 13 from a projection image engine or image projector 14 (e.g., a liquid crystal display imager, a light source, and a projection lens that produce image light in response to input video or other signals). The break in the light path of the light 13 shown in FIG. 1 is included to recognize that the light 13 may be processed or filtered, for example, projected by the projection or other lens (not shown), and is generally indicated by numeral 13A. The screen apparatus 10 and the image engine 14 are arranged such that a light beam exiting the Fresnel lens 11 is collimated, as shown by parallel rays of light 15. The collimated rays 15 pass across an air gap 16 to a matrix of glass beads 17-21 in the diffuser 12. The glass beads 17-21 are mounted upon an adhesive black mask layer 22 that is on a front surface 23 of a substrate 24 of the diffuser 12. As the collimated light rays 15 strike any of the glass beads 17-21, the rays are focused as light 25 in FIG. 1. The substrate 24 is light transparent so that a viewer 27 can see an image from the light 25 that passes through a surface 26 (e.g., an acrylic, polystyrene, other polymer or like surface) of the screen apparatus 10. The screen apparatus 10 can be an xe2x80x9cintelligentxe2x80x9d television screen, having a large diagonal dimension, for example, substantially 60 inches, or a computer monitor screen.
For wide angle projection, there are currently no satisfactory methods of collimating light at a display screen. The conventional Fresnel lens 11 may create objectionable shadows and ghosts (i.e., ghost images from light scattered in undesired directions) that degrade the display image. The discontinuities in the Fresnel lens 11 lead to shadows and ghosts being introduced on illumination of the screen apparatus 10. The formation of a ghost image from discontinuous surfaces of a Fresnel lens is schematically illustrated in FIG. 2 and its appearance on the viewing side of a Fresnel lens/diffuser combination screen is schematically illustrated in FIG. 3. For discussion on the disadvantages of such stepped lenses, see Antenna Theory by Constantine A. Balanis, Harper and Row, New York, 1982, p. 650 and Antenna Engineering Handbook, H. Jasik (ed.) (Chapter 14 by S. B. Cohn), McGraw-Hill, New York, 1961, pp. 14-1 to 14-43. Moreover, the Fresnel lens 11 is also disadvantageous because it may be relatively expensive, easily damaged, have visible rings, and cannot be laminated (e.g., index matched) on both sides. Removing the Fresnel lens 11 and relying only on the diffusive screen 12 itself to achieve uniformity may result in a xe2x80x9chot spotxe2x80x9d in the center of the screen and wasted light diffused out of the field of view of the viewer 27. The air gap 16 between the Fresnel lens 11 and the glass beads 17-21 also prevents the screen apparatus 10 from being as compact or as mechanically stable as might otherwise be possible.
The present invention is directed to overcoming or substantially limiting some or all of the above shortcomings of the Fresnel lens/diffuser combination screens, and the occurrence of the hot spot when no Fresnel lens is used.
In one aspect, the invention features a screen apparatus. The screen apparatus includes a holographic optical element adapted to receive image light and to redirect the image light. The screen apparatus also includes a diffuser adapted to receive the redirected image light from the holographic optical element and to scatter the redirected image light.